text: The PDGF family consists of PDGF-A, -B, -C and -D, which form either homo- or heterodimers (PDGF-AA, -AB, -BB, -CC, -DD). The four PDGFs are inactive in their monomeric forms. The PDGFs bind to the protein tyrosine kinase receptors PDGF receptor-α and -β. These two receptor isoforms dimerize upon binding the PDGF dimer, leading to three possible receptor combinations, namely -αα, -ββ and -αβ. The extracellular region of the receptor consists of five immunoglobulin-like domains while the intracellular part is a tyrosine kinase domain. The ligand-binding sites of the receptors are located to the three first immunoglobulin-like domains. PDGF-CC specifically interacts with PDGFR-αα and -αβ, but not with -ββ, and thereby resembles PDGF-AB. PDGF-DD binds to PDGFR-ββ with high affinity, and to PDGFR-αβ to a markedly lower extent and is therefore regarded as PDGFR-ββ specific. PDGF-AA binds only to PDGFR-αα, while PDGF-BB is the only PDGF that can bind all three receptor combinations with high affinity.
  Dimerization is a prerequisite for the activation of the kinase. Kinase activation is visualized as tyrosine phosphorylation of the receptor molecules, which occurs between the dimerized receptor molecules (transphosphorylation). In conjunction with dimerization and kinase activation, the receptor molecules undergo conformational changes, which allow a basal kinase activity to phosphorylate a critical tyrosine residue, thereby "unlocking" the kinase, leading to full enzymatic activity directed toward other tyrosine residues in the receptor molecules as well as other substrates for the kinase.
  Expression of both receptors and each of the four PDGFs is under independent control, giving the PDGF/PDGFR system a high flexibility. Different cell types vary greatly in the ratio of PDGF isoforms and PDGFRs expressed. Different external stimuli such as inflammation, embryonic development or differentiation modulate cellular receptor expression allowing binding of some PDGFs but not others. Additionally, some cells display only one of the PDGFR isoforms while other cells express both isoforms, simultaneously or separately.